JOINT BASE LEWIS-MCCHORD, WASH. — Staff Sgt. Robert Bales, the American soldier who massacred 16 Afghan civilians during pre-dawn raids in two villages last year, could soon learn whether he might ever be released from prison for a crime that even he acknowledges he can’t explain.

The Ohio native and father of two from Lake Tapps, Wash., pleaded guilty in June in a deal with prosecutors to avoid the death penalty. His sentencing begins Tuesday with the selection of a military jury to determine whether he receives life in prison with or without the possibility of release.

His lawyers have indicated they plan to present evidence that could warrant leniency, including that during at least one of his prior deployments to Iraq, Bales had been prescribed the anti-malaria drug mefloquine, known by its brand name Lariam. Last month, the U.S. Food and Drug Administration issued a new warning that the drug can cause long-term neurological damage and serious psychiatric side effects.

“Our general theme is that Sgt. Bales snapped,” said John Henry Browne, one of his civilian attorneys. “That’s kind of our mantra, and we say that because of all the things we know: the number of deployments, the head injuries, the PTSD, the drugs, the alcohol.”

Bales, on his fourth combat deployment, had been drinking and watching a movie with other soldiers at his remote post at Camp Belambay in Kandahar Province when he slipped away before dawn on March 11, 2012. Bales says he had also been taking steroids and snorting Valium provided by other soldiers.

Armed with a 9 mm pistol and an M-4 rifle, he attacked a village of mud-walled compounds called Alkozai then returned and woke up a fellow soldier to tell him about it. The soldier didn’t believe Bales and went back to sleep. Bales left again to attack a second village known as Najiban.

A total of 16 people were killed — most of them women and children — and many of their bodies were burned. Six others were injured.

The Army has flown several witnesses from Afghanistan to testify at the sentencing, but has declined to identify them. During a hearing last year, some witnesses testified by video link from Afghanistan, including a young girl in a bright headscarf who described hiding behind her father as he was shot to death. Boys told of begging the soldier to spare them, yelling: “We are children! We are children!” A thick-bearded man told of being shot in the neck by a gunman from an arm’s length away.

The massacre prompted such angry protests that the U.S. temporarily halted combat operations in Afghanistan, and it was three weeks before Army investigators could reach the crime scene.

At one point during his plea hearing, the judge, Col. Jeffery Nance, asked Bales why he killed the villagers.

Bales responded: “Sir, as far as why — I’ve asked that question a million times since then. There’s not a good reason in this world for why I did the horrible things I did.”

If he is sentenced to life with the possibility of parole, Bales would be eligible in 10 years, but there’s no guarantee he’d receive it.

Browne declined to say who might testify on Bales’ behalf at the sentencing, which is expected to last about a week. At an earlier hearing, Bales’ lawyers said those who might testify include an aunt, who could speak about any family history of mental health issues; an older brother; a principal and football coach from Norwood High School in Norwood, Ohio, where Bales grew up; and his high school football teammate Marc Edwards, who went on to become a running back on NFL teams, including the 2002 Super Bowl champion New England Patriots.

A hearing was scheduled Monday on motions related to the sentencing. Among the issues still unclear is how the judge will ensure that prosecutors make no use of compelled statements Bales gave to Army doctors. The statements are protected by Bales’ Fifth Amendment rights against self-incrimination and neither they nor any information derived from them can be used against him.